The present invention relates generally to capacitive proximity sensors. More particularly, the sensor of the invention utilizes a proximity array sensor covered with a layer of compressible non-conducting material allowing it to detect both the presence of a conductive object nearby and the force applied by such object onto the sensor surface.
Capacitive touch sensors are commonly used as an input device in consumer electronics. iPhone by Apple, Inc. for example is a widely-used device with an improved user interface made possible by using a proximity touch sensor input device. While the sensitivity of such capacitive touch sensors is very high in detecting the presence or absence of a conductive object nearby, they cannot be used effectively to measure the progressive force applied by the object onto the surface of the sensor. Proximity sensors are used in other known applications of the prior art, such as capacitive touch panels comprising a grid of electrodes formed in rows and columns as shown in FIG. 1. They provide very sensitive object detection because the sensing mechanism is in the electrical properties of the sensor using grounded or conductive object such as a finger, bare foot or metal. However, this proximity effect does not measure the pressure or the force applied by the object such as the foot. Instead, by integrating all the measurements it effectively provides the foot size rather than the person's weight—studies of scale performance have shown them to be inaccurate. Their reading also highly depends on whether the person is wearing socks or not.
Tactile array sensors on the other hand, do measure the actual contact pressures and can integrate those into contact forces. However, their performance is frequently not accurate enough for many common applications such as for example with force-sensing resistors (FSR). Another disadvantage of tactile array sensors is difficulty and complexity of their fabrication.
A cross sectional view of a tactile sensor array sensor of the prior art (U.S. Pat. No. 7,609,178 by the same inventor) is shown in FIG. 2. It includes a first electrode 101 separated from the second electrode 103 by a gap 105. Compression of electrode by an object 104 in the area 110 results in changing of capacitance between the two electrodes which is then detected by the control unit. A highly sensitive tactile sensor of this design requires a molded structure to support the ground electrode. By integrating all the pressure measurements, a total force or weight can be extrapolated. Applications like this requiring both high sensitivity and durability against large shear forces such as when a person is stepping and walking on the sensor, pose a big challenge since the ground electrode can delaminate from the mounted substrate.
The need exists therefore for a highly sensitive and reliable sensor which is easily manufactured and provides for both the accuracy of the tactile sensor and the sensitivity of the proximity sensor.